1. Field of the Invention
The present invention relates to a method for treating an optical device comprising a lithium niobate single crystal or a lithium tantalate single crystal, particularly a treatment method with a purpose of recovering functions of an optical device of which functions are decreased by photorefractive effect (optical modulation), and an optical device of which functions are recovered, obtained by the above treatment method. Further, the present invention relates to an optical frequency conversion (optical modulation) method using an optical device having polarization inversion formed thereon, which comprises generating an oscillation light having a specific wavelength and returning it to the optical device thereby to recover functions of the optical device, and an optical device set to obtain a light having a specific wavelength,
2. Discussion of Background
A lithium niobate single crystal and a lithium tantalate single crystal have high electro-optical effect and non-linear effect, whereby they have attracted attention as useful optically functional materials to be used to design various optically functional devices such as optical modulators and frequency conversion devices. In recent years, studies and development on frequency conversion devices and electro-optical devices having ferroelectric polarization inverted periodically or in a specific shape on these materials have been actively carried out.
Particularly in recent years, development as a high level optical frequency conversion device has been expected and attracted attention. For example, an attempt to achieve second harmonic oscillation to convert a laser beam in the near infrared wavelength region (fundamental wavelength: 1064 nm=1.064 xcexcm) to a green light having a half wavelength (532 nm), and an attempt to obtain a blue light by setting the fundamental wavelength and the polarization inversion periodic cycle to be shorter, and further, an attempt to achieve third harmonic oscillation to convert the above fundamental wavelength beam to a light having a wavelength one third of the fundamental wavelength (355 nm) by setting the polarization inversion periodic cycle to be very short, may be mentioned.
However, in order to positively utilize an optical device designed based on such a single crystal, xe2x80x9cphotorefractive effectxe2x80x9d has to be overcome. The photorefractive effect is defined as such a phenomenon that a refractive index change resulting from photoelectromotive force appears in the crystal when the crystal is irradiated with an intense light such as a laser beam. The result of this phenomenon is considered that as the light strikes the crystal not entirely but locally, deviation in charge density arises, whereby the crystal is naturally in such a state that an electric field is applied thereto. Namely, the phenomenon is considered to be due to a change in refractive index in the crystal by an electro-optical effect when an electric field is applied to a lithium niobate or lithium tantalate single crystal.
In any case, if an optical device undergoes such a photorefractive effect, even if frequency conversion is tried, the frequency conversion efficiency extremely decreases, and matching properties are lost, whereby oscillation may not take place, or the laser beam mode will be remarkably poor.
Accordingly, it is inevitable to overcome this problem of the photorefractive effect in order to increase usefulness of the optical device, particularly to proceed development of frequency conversion technique to obtain a coherent light based on quasi-phase-matching with polarization inversion formed, particularly oscillation technology regarding a green light and a blue light in the visible region and further, ultraviolet rays exceeding said region. Means to overcome this problem have already been published and proposed in literatures, and carried out also.
Namely, R. L. Byer, Y. K. Park, R. S. Feigelson and W. L. Kway: xe2x80x9cApplied Physics Lettersxe2x80x9d vol. 39 (1981) p. 17 discloses that an optical device comprising a single crystal is heated to from 100 to 200xc2x0 C. to increase the electric conductivity of the crystal, thereby to resolve the photorefractive effect.
Further, D. A. Bryan, R. Gerson and H. E. Tomaschke: xe2x80x9cApplied Physics Lettersxe2x80x9d vol. 44 (1984) p. 847 and Y. Furukawa, K. Kitamura, S. Takekawa, A. Miyamoto, M. Terao and N. Suda: xe2x80x9cApplied Physics Lettersxe2x80x9d vol. 77 (2000) p. 2494 disclose addition of MgO to a single crystal, and T. R. Volk, V. I. Pryalkin and N. M. Rubinina: xe2x80x9cOptics Lettersxe2x80x9d vol. 15 (1980) p. 996 discloses addition of ZnO to a single crystal, respectively, to increase the optical conductivity so that no photorefractive effect will be caused.
In association with this, the present inventors have developed a lithium niobate single crystal and a lithium tantalate single crystal, having a composition constituting the crystal boundlessly close to a stoichiometric composition, and a method for growing these single crystals, and proposed that the amount of MgO or ZnO can be considerably reduced by the method as compared with a conventional technique, and have filed a Patent Application.
However, there are still problems remaining regarding the conventional means to resolve photorefractive effect. Namely, the means of heating the crystal device to 200xc2x0 C. costs itself, and design and control of an apparatus for it are by no means easy, and adequate countermeasure is required taking impact on other equipment and apparatus including the optical frequency conversion device into consideration, and it has a drawback in view of miniaturization of the apparatus. Further, the latter means of adding MgO or ZnO can be evaluated to a certain extent from such a viewpoint that the photorefractive effect is less likely to occur as compared with a case of no addition, however, growth of a homogeneous single crystal and processing tend to be difficult, and in addition, the photorefractive effect can be by no means basically prevented with this method alone, and the means has been limited to a certain range of use.
In addition, the photorefractive effect is less likely to occur on a ferroelectric single crystal used in the present invention in a long wavelength region at the time of optical frequency conversion, whereas in the ultraviolet region with a wavelength shorter than 300 nm, the crystal itself undergo extreme degeneration, loses transparency, thereby can not be used as an optically functional material. Thus, the wavelength of the light to be used for the optical device designed to comprise such a single crystal is considerably limited, particularly in the wavelength region with a short wavelength of at most 400 nm, a remarkably intense photorefractive effect occurs, and application of light having a wavelength in this region itself, including irradiation and oscillation, is commonly considered to be unreasonable.
On the other hand, increase of a usable wavelength region has been required, and a blue light having a short wavelength for example is highly needed in fact.
Under these circumstances, the present invention has been made to overcome the above problems. Namely, the conventional means of suppressing and controlling the photorefractive effect itself is problematic and is insufficient from the above various viewpoints, and it is an object of the present invention to provide a means to easily and securely suppress the photorefractive effect with no such a problem. In addition, it is an object of the present invention to realize and achieve stably controlled optical frequency conversion and optical modulation in a short wavelength region, at which use of the optical device has conventionally been considered to be difficult, without a photorefractive effect.
The present inventors have conducted extensive studies and as a result, have basically gotten hold of the problem of the photorefractive effect. In order to discover a clue in solving the problems, they have conducted measurement regarding the wavelength dependency of the photorefractive effect. As a result, it was found that the photorefractive effect becomes more significant when the wavelength becomes shorter, under irradiation with light in a wavelength region longer than 400 nm. In experiments, a significant photorefractive effect was observed with a light of 408 nm. On the other hand, in a further shorter wavelength region, surprisingly, a remarkable increase in the photoconductivity (electric conductivity under light irradiation) of the crystal was observed. Namely, a fact which denies the concept that the optical device cannot be used at this range, is brought in this regard. From experimental results, it was found that the photorefractive effect is suppressed by irradiation with a light having a wavelength of 350 nm. From these discoveries, it was found that when a crystal or a device is uniformly irradiated with a ultraviolet light having a wavelength at a level of 350 nm, the photorefractive effect can be suppressed without heating the crystal, at the time of frequency conversion in a wavelength region longer than 400 nm, i.e. in a region in which the photorefractive effect is likely to occur.
Further, frequency conversion to an ultraviolet light by quasi-phase-matching by periodic polarization inversion of a lithium niobate single crystal or a lithium tantalate single crystal, has conventionally been considered to be difficult, since the photorefractive effect tends to be significant when a light having a short wavelength is employed. However, as a result of experiments, the present inventors have succeeded in oscillation of third harmonic wave (wavelength: 352 nm) of a fundamental light (wavelength: 1,064 xcexcm) of Nd:YAG laser, by appropriately adjusting the polarization inversion periodic cycle of the device. Namely, it was found that a short wavelength can satisfactorily be obtained.
It was further found that by allocating and reducing a part of the light having a specific wavelength (third harmonic wave) obtained by the above-described means to the device again on the spot, i.e. by returning the oscillated specific wavelength light to the device, the photorefractive effect which is gradually caused by incidence of a fundamental wave in quasi-phase-matching can self-supportingly be suppressed, that is, optical frequency conversion and optical modulation in a wavelength region in which they are hardly be carried out due to the photorefractive effect, can persistently and stably be controlled.
The present invention has been made on the basis of these important discoveries.
Namely, the present invention is to provide a means to securely resolve and prevent the photorefractive effect by irradiating an optical device with a light having a specific wavelength, based on a principle totally different from a conventional one, which is novel and useful as compared with a conventional means such as heating. The present invention further provides a method for treating a photorefractive effect of an optical device based on the above means, including optical frequency conversion, a function-recovered optical device treated thereby, a photorefractive effect-suppressed optical frequency conversion (optical modulation) method and a photorefractive effect-suppressed optical device to be used for the method.